Polycarbonates are polymeric esters of carbonic acid with diols. Because of their interesting physical properties, such as low weight, good temperature and impact resistance, and excellent optical properties, in particular, polycarbonates are used for many products in the high-tech field. As a result, there is a continuously growing demand for high-quality polycarbonate.
Polycarbonate can be produced by polycondensation of phosgene with diols, or—a process which avoids the highly toxic phosgene species—by a transesterification reaction of carbonic acid diesters with bisphenols. The production of polycarbonates by transesterification reaction of carbonic acid diesters with bisphenols is generally carried out following a melt transesterification process.
The production of polycarbonates according to the melt transesterification process above is known, and described by way of example in “Schnell,” Chemistry and Physics of Polycarbonates, Polymer Reviews, Vol. 9, Interscience Publishers, New York, London, Sydney 1964, in D.C. Prevorsek, B. T. Debona and Y. Kersten, Corporate Research Center, Allied Chemical Corporation, Morristown, N.J. 07960, “Synthesis of Poly(ester)carbonate Copolymers” in Journal of Polymer Science, Polymer Chemistry Edition, Vol. 19, 75-90 (1980), in D. Freitag, U. Grigo, P. R. Müller, N. Nouvertne, BAYER AG, “Polycarbonates” in Encyclopedia of Polymer Science and Engineering, Vol. 11, Second Edition, 1988, pages 648-718, and finally in Dres. U. Grigo, K. Kircher and P. R. Müller “Polycarbonate” in Becker/Braun, Kunststoff Handbuch, Vol. 3/1, Polycarbonate, Polyacetale, Polyester, Celluloseester, Carl Hanser Verlag, Munich, Vienna 1992, pages 117-299.
The reaction which proceeds from bisphenol and diaryl carbonates to the production of polycarbonates is represented by the following equation:

As can be seen from the above equation, a hydroxyaryl reaction product—such as phenol, for example—is released by the reaction. In the removal thereof by means of distillation, the phenol is obtained in the known method in a highly impure form, and cannot be used further without prior purification. As such, there is no possibility of using the phenol to produce new diaryl carbonate to be used again in the process, without intermediate purification steps inserted.
In the apparatuses described in the prior art used for the production of polycarbonates by melt transesterification processes, a substantial problem is that manual interventions are necessary, such as, by way of example, the cleaning of condensation units or heat exchangers, for example due to oligomeric reaction products made of the bisphenol and diaryl carbonates. This need for manual interventions leads to lengthy operation pauses with corresponding economic disadvantages. At the same time, the need for manual interventions also creates a risk for the operations personnel in the plant. As such, in cases where diphenolcarbonate (DPC) is used as the diaryl carbonate component, which is common, phenol is released as a reaction product. Due to the high toxicity and aggressiveness of phenol, every manual intervention then constitutes a potential endangerment of the operations personnel.
An important quality feature of polycarbonates is the absence, to the greatest degree possible, of discolorations. This is particularly important where polycarbonates are used for window panes, automobile headlights, and optical devices. In the apparatuses known in the prior art for the production of polycarbonate, problems consistently arise with respect to slight discolorations of the polycarbonate product, particularly in the form of yellow discolorations.